Arsenic is a naturally occurring element in the earth's curst and can be found in many natural ecosystems. Mining of arsenic containing ores often releases arsenic into the soil. Burning of arsenic containing fossil fuels, volcanic eruptions, and weathering processes also can introduce substantial amounts of arsenic into the environment. Various industrial activities such as smelting, petroleum refining, pesticide and herbicide manufacturing, and glass and ceramic production can generate arsenic containing wastewater. The presence of arsenic in natural waters may originate from geochemical reactions, industrial waste discharges or agricultural use of pesticides containing arsenic. Arsenic is typically mobile within the environment and may circulate many times in various forms through the atmosphere, water, and soil before finally entering into sediments.
Hyper-pigmentation, skin cancer, liver cancer, circulatory disorders, and other ailments have been attributed to the presence of arsenic in water. The United States Environmental Protection Agency (USEPA) has identified arsenic as a group “A” known carcinogen. This classification is based on sufficient evidence of carcinogenicity from human data involving occupational and drinking water exposures. Arsenic presents a potential health problem due to its toxicity. In response to these health concerns, the USEPA, in January 2001, promulgated the new arsenic rule that lowered the maximum contaminant level (MCL) in drinking water to 10 μg/L (10 ppb) for both community and non-transient, non community water systems. Previously, the Safe Drinking Water Act (SDWA) had a minimum arsenic standard of 50 ppb. The USEPA lowered the standard based on recommendations by the National Research Council, which reviewed scientific studies on the health effects of arsenic on human populations. According to some estimates, conventional water treatment systems will cost the nation between $180 and $725 million to meet the 10 ppb standard set by the USEPA.
Arsenic Chemistry
Arsenic often occurs in inorganic form in aquatic environments, often resulting from the dissolution of solid phases such as arsenolite (As2O3), arsenic anhydride (As2O5) and realgar (AsS2). The chemistry of arsenic in aquatic systems is typically complex because the element can be stable in four major oxidation states (+5, +3, 0 and −3) under different redox conditions. In natural waters arsenic is typically found as an anion with acid characteristics in only the As (III) and As (V) oxidation states. In oxygenated waters, the oxyanions of arsenic typically exist in four different arsenate species as H3AsO4, H2AsO42 and AsO43 in the pH ranges of <2, 3-6, 8-10 and >12, respectively. Arsenite is more likely to be found in oxygen free (anaerobic) groundwater, while arsenate is more common in aerobic surface water. Arsenite ions are typically oxidized to arsenate in the presence of oxygen, chlorine, or potassium permanganate. Therefore under neutral conditions and acidic conditions, As (III) exists as a neutral species and cannot be adsorbed by an adsorbent based on ionic interaction alone. The chemistry of arsenic is more fully described in U.S. Pat. No. 6,197,201.
Several methods for reducing arsenic concentrations to acceptable levels have been studied and are in current use. These methods include coagulation and precipitation using ferric chloride and sulfate, ion exchange, reverse osmosis, and adsorption using activated carbon and alumina. These methods are effective to a certain extent. However, these methods can be considerably more expensive, and generally narrower in application, than is typically desired for the treatment of large volumes of water. In addition, it can be difficult to implement smaller scale filtration using existing filtering techniques, such as in columns. These difficulties can make it difficult to implement point-of-use or point-of-entry filtration.
The use of ferric chloride, hydrated lime, sodium sulfate and alum to coagulate water-containing arsenic has been described. Harper et al., “Removal of Arsenic from Wastewater using Chemical Precipitation Methods”, 64(3) Water Environment Research 200-203, 1992. These methods typically require multiple treatments of water with coagulation chemicals, and large amounts of chemicals relative to the amount of arsenic present, to obtain the desired reduction in arsenic concentration. In addition, the methods typically produce sludge that requires dewatering or solidification and eventually storage in a landfill as hazardous waste. Also, the ferric chloride process typically requires a pH of less than 6.5. Merrill et al., “Field Evaluation of Arsenic and Selenium by Iron Co-precipitation”, “6(2) Environmental Progress 82-90, 1987.
A method of precipitating arsenite and arsenate ions from aqueous solutions using yttrium carbonate at alkaline pH has also been described. Wasay et al. “Removal of Arsenite and Arsenate Ions from Aqueous Solution by Basic Yttrium Carbonate”, 30(5) Wat. Res. 1143-1148, 1996. This method typically requires strict control of pH to achieve arsenic removal sufficient to comply with environmental standards. In addition, the effective pH range was found to depend on which arsenic species was desired to be precipitated.
U.S. Pat. No. 3,956,118 purports to disclose a process for removing phosphate ions from wastewater using a rare earth salt. However, the disclosed process appears to be limited to removal of phosphates.
Adsorbents, such as lanthanum oxide and lanthanum-alumina oxide, have been used for removing arsenate and arsenite species from solution, such as described in U.S. Pat. No. 5,603,838. This patent purports to disclose that lanthanum oxide alone, or in conjunction with alumina solids and other oxides, can remove arsenic to low levels (<50 ppb). The adsorption kinetics were stated to be extremely fast compared to other adsorbents such as alumina. Davis et al., “Transport Model for the Adsorption of Oxyanions of Selenium (IV) and Arsenic (V) from water onto Lanthanum and Alumina Oxide”, Journal of Colloid and Interface Science, 188, 1997, p. 340-350; Misra et al. “Adsorption of Oxyanions of Selenium onto Lanthanum Oxide and Alumina”, Minor Elements 2000, Published by SME, February 2000, pp. 345-353; Misra et al., “Adsorption and Separation of Arsenic from Process Water Using LS™ (Lanthanum-Silica Compound)”, Proceedings of the Randol Gold Forum'97, 1997, pp. 203-206; Rawat et al., “Adsorption of the Oxyanions of Arsenic onto Lanthanum Oxide”, EPD Congress, The Minerals, Metal and Materials Society (TMS), Warrendale, Pa., 1998, pp. 14-23.
A novel precipitation process developed by Misra et al. (U.S. Pat. No. 6,197,201) uses lanthanum chloride and optionally other salts to selectively co-precipitate arsenite and arsenate from process water. Misra et al., “Enhanced Precipitation and Stabilization of Arsenic from Gold Cyanidation Process”, Minor Elements 2000, Published by SME, February 2000, pp. 141-148; Nanor et al., “Removal and Stabilization of Arsenic”, Randol Gold Forum, 1999, pg. 191-196; Nanor et al., 1998. U.S. Patent Publication No. 2006/0086670 to Misra et al., describes the use of lanthanum hydroxide compositions to precipitate and remove arsenic from solutions.
General drawbacks of the processes discussed above can include inefficient removal of arsenic to an acceptably low level for drinking water and discharge into ground water, the problem of filtration of precipitated sludge, and fouling of resins and membranes. In addition, once the arsenic species are removed, the solid materials formed must typically be disposed of. The solid materials formed from the above processes also can be susceptible to leaching of the metals at a future time. Although the pre-coat process can remove arsenic from drinking water to below 10 ppb, it typically requires about 10 minutes of contact time to accomplish this. In addition, precipitate build up on the surface of the bed can reduce flow rates and require frequent cleaning or replacement of the bed.
Problems involving transportation, storage, and use of prior filtering compositions can also be encountered. For example, compositions which are maintained in a wet or highly moist state can be difficult to transport or to pack into a column or bed for use.